Lactoperoxidase, peroxide, and thiocyanate ion form an antibacterial system in saliva. The proposed research is designed to determine how this system kills or inhibits growth of bacteria. Our goal is to find ways to increase the effectiveness and selectivity of this natural defense that will be relevant to prevention of oral disease. Also, it will be determined whether the mechanism of action of the salivary system is different from that of the leukocyte antimicrobial system, myeloperoxidase, peroxide, and halide ion. Such studies may help to obtain more effective prevention of infection in other parts of the body. The chemistry of antibacterial reactions will be studied using radioactive forms of halide ions or thiocyanate. The chemical nature and cellular location of chemical modifications will be determined. These results will be correlated with studies on killing or growth inhibition, to determine which chemical reactions are essential to antibacterial action. To make more effective use of antibacterial action in the oral environment, the optimum conditions for action will be determined, and the role of bacterial cell structure and metabolism in determining susceptibility will be studied. Also, the combined action of the peroxidase systems and other agents, such as salivary components, will be studied to attempt to make antibacterial action selective against oral pathogens.